Field of the Invention
The present invention relates to a surface emitting laser and an optical coherence tomography apparatus including the surface emitting laser.
Description of the Related Art
Vertical cavity surface emitting lasers (VCSELs) are an example of surface emitting lasers. VCSELs include an active layer and two reflectors that sandwich the active layer from above and below to form a resonator in a direction perpendicular to the surface of a substrate. VCSELs emit a laser beam in the direction perpendicular to the surface of the substrate. Also, wavelength tunable VCSELs, with which the wavelength of emitted light can be varied, are known. In an example of such a VCSEL, a gap portion is provided between an upper reflector and an active layer of the VCSEL, and the wavelength of emitted light can be varied by varying the cavity length by moving the upper reflector in a direction of the optical path of the laser beam. Surface emitting lasers with which the wavelength of the emitted light can be varied are hereinafter referred to as wavelength tunable VCSELs.
It is known that wavelength tunable VCSELs are suitable for use as light sources of optical coherence tomography (OCT) apparatuses. In the case where a wavelength tunable VCSEL is used as a light source of an OCT apparatus, the tunable wavelength range of the wavelength tunable VCSEL may be increased to increase the resolution of the OCT apparatus in a thickness direction. The tunable wavelength range of the wavelength tunable VCSEL may be increased by, for example, increasing the reflectances of reflectors arranged above and below the active layer over a wide reflection range.
A wavelength tunable VCSEL in which a pair of distributed Bragg reflectors (DBRs) are arranged above and below an active layer is disclosed in IEEE Journal on Selected Topics in Quantum Electronics, Vol. 6, No. 6, Nov. 2000.
The wavelength dependency of the intensity of light emitted from a VCSEL is determined by factors such as the wavelength dependency of the gain spectrum of the active layer and the wavelength dependency of the reflectances of the upper and lower reflectors.
In an OCT apparatus including a wavelength tunable light source, it is desirable that the light output intensity of the light source does not vary by a large amount in accordance with the wavelength. Therefore, in the case where a wavelength tunable VCSEL, which is an example of a wavelength tunable light source, is included in an OCT apparatus, it is desirable that the wavelength dependency of the gain spectrum of the active layer and the wavelength dependency of the reflectances of the upper and lower reflectors may be small.
However, it is known that, in wavelength tunable VCSELs, the gain spectrum of the active layer is wavelength dependent. For example, the gain spectrum of the active layer may have a smooth convex shape in which the gain is large on the long-wavelength side of the peak wavelength. The shape of the gain spectrum of the active layer is determined by quantum mechanics, and therefore cannot be easily changed.
In the case where such a wavelength tunable VCSEL is used, the wavelength dependency of the light output intensity may be large, and there is a risk that accurate OCT images cannot be obtained.